In recent years, a light-emitting element (also referred to as an electroluminescent (EL) element) in which a light-emitting layer (hereinafter also referred to as an EL layer) containing an organic compound is interposed between a pair of electrodes has been actively developed. Lighting has attracted attention as one of application fields of the light-emitting element. This is because a lighting device including the EL element has features different from those of other lighting devices; for example, the lighting device including the EL element can be thin and light, and perform surface emission.
As described above, an EL element has a structure in which a light-emitting layer containing an organic compound is interposed between a pair of electrodes. Thus, emission from the light-emitting layer is extracted through at least one of the pair of electrodes. Accordingly, in general, at least one of the pair of electrodes in the EL element is formed using a conductive film having a property of transmitting visible light (a transparent conductive film).
However, resistivity of a transparent conductive film is one or two orders of magnitude higher than that of metal which easily conducts electricity. In particular, in a large-area element that is used for a lighting purpose or the like, there is a problem in that change in luminance in a light-emitting region of the light-emitting element due to voltage drop is significant. Therefore, Patent Document 1 suggests that a metal layer formed using a substance whose resistance is lower than that of the transparent conductive film is provided between the transparent conductive film and the light-emitting layer containing an organic compound (see Patent Document 1).
There is another problem in that under the circumstances, the cost for manufacturing a lighting device including an EL element is very high. The high manufacturing cost results in a high price of the lighting device, which deteriorates the competitiveness of the lighting device including an EL element even though it has the above-described features different from those of other lighting devices. In order to spread the lighting device including an EL element, cost reduction in terms of both a material and a process is needed.